Drag and drop images in the editor to import and insert (#5699)

* Merge

* Clean up

* More clean up

* Ensure image import works when type is not set, clean up post import actions

* Removed spurious new line

* For non image files insert a tiddler link

* Added documentation for new settings and features

core/modules/utils/dom/dragndrop.js

@@ -205,4 +205,16 @@ function parseJSONTiddlers(json,fallbackTitle) {
 	return data;
 };
 
+exports.dragEventContainsFiles = function(event) {
+	if(event.dataTransfer.types) {
+		for(var i=0; i<event.dataTransfer.types.length; i++) {
+			if(event.dataTransfer.types[i] === "Files") {
+				return true;
+				break;
+			}
+		}
+	}
+	return false;
+};
+
 })();